The present invention relates to ultrasonic motors and methods for manufacturing ultrasonic motors, and, more particularly, to structures for damping undesirable vibration of ultrasonic motors.
A typical ultrasonic motor has a stator and a rotor. The stator includes an annular vibrating portion at the outer periphery of the stator. An annular piezoelectric element is secured to the vibrating portion. A pressing member presses the rotor against the stator. A rotary shaft is connected to the rotor and rotates integrally with the rotor. When provided with high frequency voltage, the piezoelectric element vibrates to produce progressive waves in the vibrating portion of the stator. This rotates the rotor integrally with the rotary shaft.
Generally, an external device is connected to the rotary shaft of the motor and applies load to the motor. The load acting on the motor rapidly changes if the motor is started or stopped. This produces a sudden vibration between the external device and the rotary shaft or between the rotary shaft and the rotor. The vibration is transmitted to the vibrating portion of the stator through the rotor and affects the progressive waves in the vibrating portion. This leads to various problems such as unstable rotation of the motor, reduced motor power, and noise.
Japanese Unexamined Patent Publication No. 7-250488 describes an ultrasonic motor that solves these problems. More specifically, the ultrasonic motor includes an annular groove formed along an outer periphery of a rotor. The groove opens radially inward, and an elastic, rubber ring is fitted in the groove. The ring damps undesirable vibration otherwise transmitted from the rotor to a stator, thus optimizing the rotation of the motor.
However, although the aforementioned publication includes the elastic ring in the rotor, it does not specify a detailed structure of the ring. Thus, to reliably damp undesirable vibration, it is necessary to specify the structure of the elastic ring.
Accordingly, it is an objective of the present invention to provide an ultrasonic motor that reliably damps undesirable vibration and a method for manufacturing the ultrasonic motor.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, the invention provides an ultrasonic motor comprising a stator, which includes a piezoelectric element, a rotor, which is opposed to the stator, wherein the piezoelectric element vibrates the stator to rotate the rotor, and an elastic member secured to the rotor, wherein the mass of the elastic member is within a predetermined range of masses, and the motor speed at which a predetermined level of noise is produced by the motor is substantially constant for the masses in the predetermined range.
The invention also provides an ultrasonic motor comprising a stator, which includes a piezoelectric element, a rotor, which is opposed to the stator, wherein the rotor includes an annular contact portion that contacts the stator and an annular thin section, which is located radially inward with respect to the contact portion, the thin section being thinner in the axial direction than the remainder of the rotor, wherein the piezoelectric element vibrates the stator to rotate the rotor, and an annular elastic member, which is secured to the thin section, the specific gravity of the elastic member being equal to or smaller than 1.45, and the thickness of the elastic member in the axial direction being 1.25 or more times as large as the thickness, as measured in the axial direction, of the thin section.
The invention further provides a method for manufacturing an ultrasonic motor comprising providing a stator having a piezoelectric element, placing a rotor in opposition to the stator such that the piezoelectric element vibrates the stator to rotate the rotor, securing an elastic member to the rotor, and selecting a mass of the elastic member from a predetermined range of masses, wherein the motor speed at which a predetermined level of noise is produced by the motor is substantially constant for the masses in the predetermined range.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.